1. Field of the Invention
The present invention relates to a resolution converting process for outputting high-resolution image signals in a low-resolution image output device.
2. Description of the Related Art
In image processing devices such as a printer, a copier, and a multi function peripheral (MFP), a Page Description Language (PDL) signal is expanded to bitmap data by the device controller. The bitmap data is output in a resolution equal to the resolution at expansion. For example, data expanded to 600 dpi can be output by using a 600 dpi printer.
Also, even when the controller receives a signal which has been expanded to bitmap data, the signal is output in a resolution equal to the resolution of the received bitmap data. In another technique, the PDL signal is expanded to 300 dpi bitmap data, the data is then doubled for output by a 600 dpi printer, in order to increase bitmap expansion speed.
In these known arts, the resolution of the expanded PDL signal is equal to or lower than the resolution of the printer. Therefore, it is difficult to obtain image quality of higher resolution than the resolution of the printer.
On the other hand, a technique for expanding PDL data to bitmap data in a resolution higher than that of the printer has been proposed. In this technique, for example, PDL data is expanded to 1200 dpi bitmap data, and each pixel of the expanded data is output by a 600 dpi printer using spot multiple-layered technology. In this way, image quality equivalent to 1200 dpi can be expressed by using a 600 dpi printer. Spot multiple-layered technology is disclosed in U.S. Pat. No. 5,134,495, for example. Also, spot multiplexing can be realized by converting pulse width so that a plurality of dots are formed in a basic cell of input data to be printed. This technique is disclosed in U.S. Pat. No. 5,109,283 and JP Patent Publication No. H04-336859.
In the above-described techniques, high-resolution data has to be expanded in a controller. For example, a signal expanded to 1200 dpi has to be (A) compressed, (B) spooled, (C) expanded, and (D) image processed, thus the controller needs to have a spec four times as large as the spec for processing 600 dpi data.
Further, problems can occur in (D) image processing. For example, when 1200 dpi data is processed with a line memory, the line memory capacity must be twice as large as that used for processing 600 dpi data. Another disadvantage is in the error diffusion process for image processing. In order to perform the error diffusion process, the errors of some lines of input image data has to be held. This also causes an increase in memory capacity requirements. For example, the memory capacity required to process 1200 dpi data is twice larger than that for 600 dpi in the main scanning (X direction).
On the other hand, when 1200 dpi data is processed in a spec for 600 dpi, the processing speed decreases, or processing cannot be executed in some cases. Therefore, these methods are not commercially practical.
In addition, when resolution conversion is performed by using spot multiple-layered technology, and the controller outputs image data to an image output device using electrophotography, image processing which consider non-linear characteristics of electrophotography cannot be performed. The reason for this is that there is no choice but to uniformly convert the image data to 600 dpi in a printer engine (laser controller) when the spot multiple-layered technology is used. Herein, uniform conversion means that a product-sum operation process is performed by using a predetermined weighting coefficient without consideration of input levels. That is, since the predetermined coefficients are the fixed values without distinction of input signal levels, uniform conversion is performed.